AlN thin film was coated over Cu substrate (575 mm2) with 400 nm thickness using DC sputtering for thermal interface material (TIM) application. Aluminum Nitride (AlN)-coated Cu substrate (AlN/Cu) was used as a heat sink for 3-W green light emitting diode (LED). The thermal transient curve was recorded for given LED attached with bare Cu and AlN-coated Cu substrate at three different driving currents. LED attached on AlN/Cu showed the reduced raise in junction temperature (TJ) by 2.59 °C at 700 mA. The LED/TIM/AlN/Cu boundary condition was not supported to reduce the TJ. The total thermal resistance (Rth-tot) was reduced for AlN-coated Cu substrate at 350 mA. The thermal resistance between metal core printed circuit board and Cu substrate (Rth-b-hs) was also observed as low for AlN-coated Cu substrates compared with other boundary conditions measured at 700 mA. The observed results were supported for the use of AlN thin film as TIM in high power LEDs.